CHAPTER 5 Generation and Conduction of Action Potentials 71 •Fig. 5.5 A Model of the Voltage-Gated Na+ Channel. A, Thelargeredelementsrepresentthefourα subunitsandthetwoyellowelementsareβ subunitswiththereceptorsitesforα scorpiontoxin(ScTX)andtetrodotoxin(TTX)indicated.B, β1andβ2subunitsflankinganα subunitareshownwiththeirtransmembranehelices.(RedrawnfromCatterallWA:Structureandfunctionofvoltage-gatedsodiumchannelsatatomicresolution.Exp Physiol. 2014;99:35-51.) associated with the action potential. Detailed statistical analyses of these recordings also allowed remarkable inferences to be made about the nature of the channels that passed these Na+ and K+ currents. The development of the patch recording, however, enabled direct observation of the behavior of individual channels. In this technique, a specially shaped microelectrode is placed against the surface of a cell, and suction is applied to the microelectrode. As a result, a high-resistance seal is formed between the mem brane and the tip of the microelectrode (Fig. 5.6A ), which allows recording of the activity of whatever channels happen to be in the patch of membrane that is inside the seal. Under ideal conditions only one or a few ion channels of a single type are present in the membrane patch.
Patch recordings show that many ion channels flip spontaneously between open and closed conductance states as if they have gates that open and close the entrance to their pore. In the case of voltage-gated channels, the gate is sensitive to the voltage across the membrane, and thus the time a gate spends in each state is a probabilistic function of the membrane potential. A patch recording of a K+ channel demonstrates this probabilistic behavior (see
Fig. 5.6B ). As the membrane potential is clamped to more depolarized levels, the channel spends more time in its open state, which reflects the voltage dependence of the probability that the channel will open (see
Fig. 5.6C ). Also, the amplitude of the current in the open state increases with the level of depolarization; this is because the driving force for K+ is greater at more depolarized levels (i.e., the membrane potential is farther from the K+ Nernst potential).
The behavior of a Na+ channel is more complex than that of the K+ channel. Like the K+ channel, it has a 72 SECTION2Berne & Levy Physiology X˜X˜X˜X˜X˜Cytosol Pipette solution Patch pipette Ground Operational amplifier Feedback resistor Micropipette Cell membrane Ion channel A 1.0 0.8 0.6 0.4 0.2 C Po °80 °60 °40 °20 200 mV D Time (1ms/division)B ˜°°50 0 50 Membrane potential (mV) °1.0 0 Na+ current density (mA/cm2)